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and
- allyl nickel
complex. Most reactions were carried out with chloride as leaving group, which gave
better yields as the acetate and the ether.
In the absence of a chiral ligand, the
γ
were obtained, clearly indicating that the reaction proceeds via a
π
-substitution product was obtained exclusively
(entry 1). As expected, the bidentate ligands were ineffective (entries 2 and 3). In con-
trast, the monodentate ferrophite ligands gave rise to the desired
α
γ
- product with up to
94% ee, although the
selectivity was low in most cases.
In 1998, Hoveyda et al. reported a detailed investigation of Grignard additions to
cyclic unsaturated acetals (Table 8B.27) especially on the infl uence of phosphine ligands
on the stereochemical outcome of the reaction [200]. The reactions proceeded with
excellent regiocontrol (
α
/
γ
98%) giving rise to substituted enol ethers, which could be
hydrolyzed under mild acidic conditions to the corresponding ketones. While the reac-
tion was slow in the presence of an Ni-PPh
3
complex (entries 1 and 2), the corresponding
dppe complexes gave the coupling products in good to excellent yield (entries 3-7). To
fi gure out if the reaction can be carried out in an enantioselective fashion, they screened
various chiral bidentate ligands. To minimize the background reaction (entries 1 and 2),
chiral nickel complexes with (
S
,
S
) -
CHIRAPHOS
(
L34
) were probed and gave highly
interesting results.
While the
in situ
-prepared chiral catalyst gave only a very moderate selectivity in the
reaction of the fi ve-membered dimethyl acetal (entry 8), the preformed chiral Ni complex
gave the coupling product with 53% ee (entry 9). Surprisingly, if the substrate with the
six-membered ring was used, the opposite effect was observed (entries 10 and 11). The
lower selectivity for the former substrate can be explained by a dominant background
>
TABLE 8B.27. Allylic Alkylations Using Cyclic Unsaturated Acetals
R
R
O
O
O
H
3
O
+
R'MgX
Ni-catalyst (5 mol %),
THF, 22°C, 1-3 h
n
n
R'
Entry
R or R,R
n
R ′ MgX
Catalyst
Yield (%)
ee (%)
1
C H
3
1
EtMgCl
(PPh
3
)
2
NiCl
2
2 5
—
2
C H
3
2
EtMgCl
(PPh
3
)
2
NiCl
2
< 5
—
3
C H
3
1
PhMgBr
dppeNiCl
2
7 0
—
4
C H
2
CH
2
1
PhMgBr
dppeNiCl
2
8 6
—
5
C H
2
CH
2
2
PhMgBr
dppeNiCl
2
6 0
—
6
C H
2
CH
2
2
EtMgCl
dppeNiCl
2
9 0
—
7
C H
3
2
PhMgBr
dppeNiCl
2
9 2
—
8
C H
3
1
EtMgCl
(PPh
3
)
2
NiCl
2
/
L34
76
15
9
C H
3
1
EtMgCl
L34
NiCl
2
85
53
10
CH
3
2
EtMgCl
(PPh
3
)
2
NiCl
2
/
L34
96
70
11
CH
3
2
EtMgCl
L34
NiCl
2
85
11
12
CH
3
2
EtMgBr
(PPh
3
)
2
NiCl
2
/
L34
90
85
